During operation of magnetic disk devices, a sector of a user region may fall in the situation that normal data read is unfeasible. A magnetic disk device detects such a sector as a defective sector. Upon finding a writing destination of data as a defective sector, the magnetic disk device writes the data to a preset alternative region in place of the user region. With no free region in the alternative region, the magnetic disk device prohibits reception of new data. In other words, in the case of no free region in the alternative region, a host cannot write new data to the magnetic disk device.